Update dependencies and improve CI workflow

[tagliala]

• Use strings instead of floats for uniformity
• Rename "deps" with "rails" for uniformity with Inherited Resources and Active Admin gems
• Update bundler
• Fix platforms

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Backported from 7.2

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.32%. Comparing base (41ebc98) to head (37e5bb9).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #600 +/- ## ======================================= Coverage 94.32% 94.32% ======================================= Files 17 17 Lines 458 458 ======================================= Hits 432 432 Misses 26 26 ```

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